We investigate the effects of spin-polarized leads on the Kondo
physics of a quantum dot using the numerical renormalization
group method. Our study demonstrates in an unambiguous way that
the Kondo effect is not necessarily suppressed by the lead
polarization: While the Kondo effect is quenched for the
asymmetric Anderson model, it survives even for finite
polarizations in the regime where charge fluctuations are
negligible. We propose the linear tunneling magnetoresistance
as an experimental signature of these behaviors. We also report
on the influence of spin-flip processes.